A magnetic disk drive includes circular data tracks on a rotating magnetic disk and read and write heads that may form a merged head attached to a slider on a positioning arm. During a read or write operation, the merged head is suspended over the magnetic disk on an air bearing surface (ABS). The sensor in a read head is a critical component since it is used to detect magnetic field signals by a resistance change. There is a magnetoresistance effect produced by spin valve magnetoresistance (SVMR) or giant magnetoresistance (GMR) which is based on a configuration in which two ferromagnetic layers are separated by a non-magnetic conductive layer in the sensor stack. One of the ferromagnetic layers is a pinned layer in which the magnetization direction is fixed by exchange coupling with an adjacent anti-ferromagnetic (AFM) or pinning layer. The second ferromagnetic layer is a free layer in which the magnetization vector can rotate in response to external magnetic fields. The rotation of magnetization in the free layer relative to the fixed layer magnetization generates a resistance change that is detected as a voltage change when a sense current is passed through the structure. In a CPP configuration, a sense current is passed through the sensor in a direction perpendicular to the layers in the stack. Alternatively, there is a current-in-plane (CIP) configuration where the sense current passes through the sensor in a direction parallel to the planes of the layers in the sensor stack.
Ultra-high density (over 100 Gb/in2) recording requires a highly sensitive read head. To meet this requirement, the CPP configuration is a stronger candidate than the CIP configuration which has been used in recent hard disk drives (HDDs). The CPP configuration is more desirable for ultra-high density applications because a stronger output signal is achieved as the sensor size decreases, and the MR ratio is higher for a CPP configuration. Furthermore, in U.S. Pat. No. 5,627,704, a GMR-CPP transducer is described that has a plurality of GMR structures which are connected serially to provide a larger output signal than can be obtained with a single GMR stack.
In the CPP GMR head structure, a bottom synthetic spin valve film stack is generally employed for biasing reasons as opposed to a top spin valve where the free layer is below the spacer and the pinned layer is above the copper spacer. Additionally, a CoFe/NiFe composite free layer is conventionally used following the tradition of CIP GMR improvements. An important characteristic of a GMR head is the MR ratio which is dR/R where dR is the change in resistance of the spin valve sensor and R is the resistance of the spin valve sensor before the change. A higher MR ratio is desired for improved sensitivity in the device and this result is achieved when electrons in the sense current spend more time within the magnetically active layers of the sensor. Interfacial scattering which is the specular reflection of electrons at the interfaces between layers in the sensor stack can improve the MR ratio and increase sensitivity.
Toshiba has shown (Ref. 3) that for a synthetic anti-parallel (SyAP) pinned layer configuration, laminating the CoFe AP1 layers with thin Cu layers can improve the MR ratio in CPP GMR heads. The resulting CPP-GMR bottom spin valve is represented by seed/AFM/pinned/spacer/free/cap where seed is a seed layer, the spacer is a copper layer, the free layer is a CoFe/NiFe composite, and the pinned layer has an [AP2/coupling/AP1] SyAP configuration in which Ru is the coupling layer and the AP1 layer is a [CoFe_Cu] laminated layer.
U.S. Pat. No. 5,715,121 discloses a further means of CPP-GMR improvement by inserting a confining current path (CCP) layer in the copper spacer by segregating metal path and oxide formation. Moreover, a soft magnetic film (free layer) is described that is Ni-rich and includes Co such as Ni0.80CO0.15Fe0.05 and Ni0.68Co0.20Fe0.12 or has a high Co content as in CO0.9Fe0.1 and CO0.7Ni0.1Fe0.2.
In a CPP operation mode, a tunnel magnetoresistive (TMR) head is another candidate for realizing high sensitivity. In this design, the non-magnetic conductive layer between the pinned layer and free layer in the GMR stack is replaced by an insulating layer such as AlOx. When the magnetoresistive element is a tunneling magnetic junction (TMJ), the tunneling (insulating) layer may be thinned to give a very low RA (<5 ohms-μm2).
A CPP-GMR head is generally preferred over a TMR head design for ultra-high density recording because the former has lower impedance. However, the resistance (RA) in a conventional single spin valve is too small (<100 mohm-μm2) and the MR ratio of a CPP head may be very low (<5%). Additionally, the output voltage which is related to the resistance change is unacceptably low for many CPP-GMR configurations. One way to increase the resistance change is to optimize the materials and structure of the CPP-GMR head.
Desirable properties for the free layer in a magnetoresistive element include low coercivity (magnetic softness) of <100e and a low magnetostriction (λs) on the order of 1×E-8 to about 5×10E-6 to reduce stress induced anisotropy. A trend in the industry is to employ high spin polarization materials such as CoFe in which the atomic % of Fe is >20%, or NiFe in which the atomic % of Fe is >50%, or [(CoFe)0.8B0.2] with ≧25 atomic % Fe in the CoFe composition in order to produce a higher MR ratio. However, higher spin polarization in a ferromagnetic layer is normally associated with a high saturation magnetization (Ms) that leads to unacceptably high (λs) and Hc values. A composite free layer with a Co90Fe10/Ni82.5Fe17.5 configuration is commonly used in CPP-GMR heads due to its small Hc (˜50e) and low λs of ˜2×10E-6 but its dR/R ratio is less than 10% and is not large enough for advanced applications. Therefore, an improved magnetoresistive element is needed that has a high MR (dR/R) ratio of at least 10%, low coercivity, and a low λs value less than about 5×10E-6.
U.S. Pat. No. 6,888,707 discloses an improved free layer comprised of a very thin CoFe/NiFe composite layer. The Fe content in the CoFe layer is about 10 atomic % and the Fe content in the NiFe layer is about 19% which provides low Hc and λs values. However, the dR/R ratio is not large enough for ultra high density recording purposes.
In U.S. Pat. No. 6,519,124 and U.S. Pat. No. 6,529,353, a NiFe/CoFe free layer is mentioned but the atomic % of Fe in each layer is not specified. Therefore, the patents do not teach how to resolve the high λs that would result from a high spin polarization CoFe component in the free layer.
U.S. Patent Application Publications 2004/0047190 and 2003/0197505 describe a Ni rich NiCoFe free layer wherein the Ni content is 60 to 90 atomic %. This layer is not used in combination with a CoFe lower layer and is designed as a “soft” magnetic layer.
In U.S. Patent Application Publication 2004/0091743, a composite free layer comprised of an upper NiFe(13.5%) layer and a lower CoFe(16%) layer is disclosed. The free layer is optimized for a slightly negative magnetostriction and does not take into account dR/R which is expected to be low because of the low Fe atomic %.